This PR adds initial support for running Consul Connect Ingress Gateways (CIGs) in Nomad. These gateways are declared as part of a task group level service definition within the connect stanza.

```hcl
service {
  connect {
    gateway {
      proxy {
        // envoy proxy configuration
      }
      ingress {
        // ingress-gateway configuration entry
      }
    }
  }
}
```

A gateway can be run in `bridge` or `host` networking mode, with the caveat that host networking necessitates manually specifying the Envoy admin listener (which cannot be disabled) via the service port value.

Currently Envoy is the only supported gateway implementation in Consul, and Nomad only supports running Envoy as a gateway using the docker driver.

Aims to address #8294 and tangentially #8647